Detection and measurement of gas concentrations using optical absorption of gas molecules is important for both understanding and monitoring a variety of phenomena from industrial processes to environment changes. Although semiconductor and electrochemical gas sensors can be highly sensitive at the low ppm level, they suffer from drift and cross-respond to other gases and changing humidity levels, along with large size and high cost. In contrast, gas sensors based on optical absorption offer, for example, 1) minimal drift as measurements are self-referenced and 2) high gas specificity with zero cross-responses to other gases, as the transduction method makes a direct measurement of a molecule's physical properties (i.e., its absorption at a specific wavelength, or so-called fingerprint wavelength). Therefore, selective detection and multiplexing are some of the attractive features offered by the optical approached.
However, most conventional optical gas sensors are commonly bulky in size and costly, while those compact and inexpensive sensors tend to lack wavelength selectivity or are less sensitive. For example, a conventional optical gas sensor based on either free space optics or fiber optics typically has a relatively large size of around 20 mm in diameter and 18 mm in height, which makes it difficult to realize chip-level integration solution. Furthermore, in conventional optical gas sensors, significant costs are attributed to complex assembling solution and expensive components of light source, detectors and filters.
Gas detection by using photonics waveguide is a promising approach due to its conspicuous advantages, such as ultra-small footprint, flexible integration with conventional electronic integrated circuits, and high operation speed. However, conventional Silicon-on-Insulator (SOI)-based waveguides are not suitable for various gas detections because its waveguide transmission window (transparency) for light propagation is relatively narrow. For example, the longest wavelength that can be supported by a conventional SOI-based waveguide may be about 3.7 μm (based on a waveguide propagation loss less than 2 dBcm−1), but the fingerprint absorption wavelengths of a large variety of gas molecules have longer wavelengths. Therefore, it is generally understood in the conventional art that waveguides based on SOI platform is not suitable to be implemented in a gas sensor. On the other hand, waveguides based on Silicon-on-Sapphire (SOS) involve high fabrication costs while not significantly improving the waveguide transmission window for light propagation.
A need therefore exists to provide an optical waveguide structure and an optical gas sensor that seek to overcome, or at least ameliorate, one or more of the deficiencies of conventional optical waveguide structures and optical gas sensors, such as significantly improving the waveguide transmission window (transparency) for light propagation. It is against this background that the present invention has been developed.